The invention relates to a process for the treatment of sodium-nitrate-containing electrolytes during the electrochemical machining (ECM) of the surfaces of iron-containing work pieces. The invention is also concerned with an apparatus for implementing said process.
In electrochemical machining (ECM), e.g., in electrochemical metal working or in electrochemical honing of the working surfaces of the cylinders of internal combustion engines, sodium nitrate (NaNO.sub.3) solvents are normally used as electrolyte. During the anodic metal dissolution the nitrate is reduced into nitrite according to the following formula: EQU NO.sub.3.sup.(-) +2[H].fwdarw.NO.sub.2.sup.(-) +H.sub.2 O
The increasing nitrite content in the electrolyte resulting therefrom forms on the surface of the workpiece to be eroded passive layers that more or less reduce the efficiency of the electrochemical machining. Accordingly, with constant current density, the surface erosion diminishes in an indefinite manner.
To some extent this can be compensated for by increasing the current density or the specific power input, but one has to reckon with another drawback in the form of an overheating of the electrolyte that, under certain conditions, requires appropriate cooling devices. In addition, more oxygen is generated at the anode and more hydrogen at the cathode, which is likewise a disadvantage for the machining efficiency.
As an example, to prevent the formation of oxygen and hydrogen, e.g., during electrochemical metal working, the electrochemical metal working, the electrochemical machining process is carried out in a pressure vessel. Moreover, the process is carried out at a higher electrolyte velocity, which requires corresponding electrolyte pressures and pump capacities and which leads to greater abrasion at the carbon electrodes.
The nitrite content of the metal sludge carried away from the surface of the workpiece and separated from the electrolyte must be detoxified, so that the sludge can be carried to a dumping ground. During the oxidation process, the sludge can be oxidized up to a higher oxidation state by adding sodium hypochloride or hydrogen peroxide. In this case, the pH value must lie between 3 and 4. In an alternative reduction process in the weak-acid region, the nitrite is reduced into nitrogen with amidosulphuric acid or urea, resulting in the production of sulphuric acid or carbonic acid.
In both processes, a flocculant must be added in order better to coagulate the iron hydroxide sludge. Therefore, in order to detoxify the nitrite, both processes require a large amount of equipment and, hence, considerable investment capital. Also, there are significant day-to-day expenses for chemicals.
The primary object of the invention is to provide a process that enables an electrochemical erosion with higher efficiency and better control and with which the metal sludge can be eliminated economically. The invention is also concerned with a particularly effective and cost-efficient apparatus for carrying out the process.
A surprising fact found by the inventor was that by continuously adding an oxidizing agent (preferably ozone (03)) the nitrite can be oxidized into a nitrate in the basic region. As a result, during the electrochemical machining, the formation of passive layers on the surface of the workpiece being processed is prevented, thus improving the machining efficiency. This results in a higher removing capacity and shorter machining times.
Furthermore, the continuous addition of the oxidizing agent causes colloidally dissolved iron-II-hydroxide to be oxidized into precipitatable iron-III-hydroxide (metal sludge). This results in better flocculation of the sludge at a higher rate of descent, so that the expenditure for the disposal of toxic waste from the installation is reduced. Therefore, the addition of flocculants can be dispensed with.
Finally, less gas (O.sub.2 +H.sub.2) is generated because of the low nitrite content in the electrolyte and the resultant reduced passive layer formation on the workpiece. Thus, one can work with lower electrolyte current velocities and, if carbon electrodes are used, this leads to less abrasion and, hence, to longer electrode residence times.
The oxidizing agent can be added to effect the advantages above and, in the case of a high metal removal capacity, an addition in the main current of the electrolyte has proved to be beneficial.
The electrolyte is treated until the mean content of the electrolyte in the oxidizing step is lowered sufficiently. Preferably, nitrite content of the removed iron hydroxide sludge separated from said electrolyte drops below about 20 mg/l. The metal sludge then can be carried--with due regard to current legal obligations--to the dumping ground without special detoxification.
According to the features discussed above, the passivation layer on the surface of the workpiece can to a certain extent be controlled, which can be beneficial with respect to a well-defined surface structure of the workpiece to be machined--particularly during electrochemical friction plating of the working surfaces of cylinders as disclosed in Unexamined West German Application No. 31 19 847.
The invention includes an apparatus for solving the problems discussed above. Thus, the continuous oxidation of the nitrite into a nitrate in the electrolyte can be effected by means of a processing station switched into the electrolyte circuit. Such a processing station can, for example, also be retrofitted in older installations.
The processing station can be constructed or arranged to inject an oxidizing agent into the electrolyte at the proper place in the circuit. The generation of ozone can be brought about by radio discharge. An intensive, continuous reaction of the ozone with the nitrite to form a nitrate occurs in the reactor. Gas evolution of the metal sludge is prevented by separating the metal sludge ahead of the processing station (reactor), allowing the metal sludge to settle quickly.
Further details of the invention will become apparent from a consideration of the ensuing description of a specific embodiment of the invention.